The impact of fluid and mineral composition on experimental carbonate silicification

The pseudomorphic replacement of carbonate minerals by silica is an omnipresent diagenetic process. However, the parameters that influence the silicification process are incompletely understood and experimental studies are sparse. Here, we present the results of batch experiments at normal conditions designed to replace various carbonates (calcite, aragonite, dolomite, magnesite, and marble) by amorphous silica. The experiments were conducted at variable pH (2–7), dissolved silica concentration (100–400 ppm), and duration ranging from 7 to 21 days. We used a multi-method analytical approach with XRPD, EPMA, Raman spectroscopy, and SEM to characterize the parent and product phases and the rich array of replacement features. After 21 days at pH 2–3, calcite and aragonite dissolve, disordered amorphous silica particles (<1 µm) precipitate, and an up to 150 µm thick replacement rim forms. During the initial calcite replacement phase, porous honeycomb-like structures form and relict rhombs remain. In a second phase, residual dissolved silica precipitates inside pore space at the replacement interface and forms dense networks of amorphous silica nanoparticles. Dense silica aggregates also form during the pseudomorphic replacement of fibrous aragonite. At the same experimental run conditions, neither dolomite nor magnesite show dissolution or precipitation features. Our results show that at low pH rim thickness increases with time and reaches its maximum at initial pH 2.5. At dissolved silica concentrations <200 ppm, no precipitation of amorphous silica was observed. The pseudomorphic carbonate-silica replacement proceeds via an interface-coupled dissolution-precipitation mechanism, in which carbonate dissolution induces silica precipitation.